scholarly journals Structure and internal deformation of thrust sheets in the Sawtooth Range, Montana: insights from anisotropy of magnetic susceptibility

2019 ◽  
Vol 487 (1) ◽  
pp. 189-208 ◽  
Author(s):  
Dave J. McCarthy ◽  
Patrick A. Meere ◽  
Michael S. Petronis
Keyword(s):  
Magnetic Susceptibility ◽  
Strain Analysis ◽  
Anisotropy Of Magnetic Susceptibility ◽  
North American Continent ◽  
The North ◽  
Magnetic Fabrics ◽  
Structural Regime ◽  
Thrust Sheets ◽  
Internal Deformation ◽  
East West

AbstractGeological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques. In the absence of valid strain markers, it can be difficult to identify the presence of an early tectonic fabric development and resulting layer parallel shortening (LPS). In order to identify early LPS, we carried out anisotropy of magnetic susceptibility (AMS) analyses on Mississippian limestones from the Sawtooth Range of Montana. The Sawtooth Range is an arcuate zone of north-trending, closely spaced, west-dipping, imbricate thrust sheets that place Mississippian Madison Group carbonates above Cretaceous shales and sandstones. This structural regime is part of the cordilleran mountain belt of North America, which resulted from accretion of allochthonous terrains to the western edge of the North American continent.Although the region has a general east–west increase in thrust displacement and related brittle deformation, a similar trend in penetrative deformation or the distribution of tectonic fabrics is not observed in the field or in the AMS results. The range of magnetic fabrics identified in each thrust sheet ranges from bedding controlled depositional fabrics to tectonic fabrics at a high angle to bedding.

Anisotropy of magnetic susceptibility (AMS) of impact melt breccia and target rocks from the Dhala impact structure, India

2021 ◽  
Author(s):  
Anuj Kumar Singh ◽  
Jayanta Kumar Pati ◽  
Shiva Kumar Patil ◽  
Wolf Uwe Reimold ◽  
Arun Kumar Rao ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Impact Structure ◽  
Magnetic Phases ◽  
Impact Melt ◽  
Rock Samples ◽  
Average Value ◽  
Magnetic Fabrics ◽  
Target Rock ◽  
The Impact

ABSTRACT The ~11-km-wide, Paleoproterozoic Dhala impact structure in north-central India comprises voluminous exposures of impact melt breccia. These outcrops are discontinuously spread over a length of ~6 km in a semicircular pattern along the northern, inner limit of the monomict breccia ring around the central elevated area. This study of the magnetic fabrics of impact breccias and target rocks from the Dhala impact structure identified a weak preferred magnetic orientation for pre-impact crystalline target rocks. The pre- and synimpact rocks from Dhala have magnetite and ilmenite as common magnetic phases. The distributions of magnetic vectors are random for most impact melt breccia samples, but some do indicate a preferred orientation. Our anisotropy of magnetic susceptibility (AMS) data demonstrate that the shape of susceptibility ellipsoids for the target rocks varies from prolate to oblate, and most impact melt breccia samples display both shapes, with a slight bias toward the oblate geometry. The average value for the corrected degree of anisotropy of impact melt rock (P′ = 1.009) is lower than that for the target rocks (P′ = 1.091). The present study also shows that both impact melt breccia and target rock samples of the Dhala structure have undergone minor postimpact alteration, and have similar compositions in terms of magnetic phases and high viscosity. Fine-grained iron oxide or hydroxide is the main alteration phase in impact melt rocks. Impact melt rocks gave a narrow range of mean magnetic susceptibility (Km) and P′ values, in contrast to the target rock samples, which gave Km = 0.05–12.9 × 10−3 standard international units (SI) and P′ = 1.036–1.283. This suggests similar viscosity of the source magma, and limited difference in the degrees of recorded deformation. Between Pagra and Maniar villages, the Km value of impact melt breccias gradually decreases in a clockwise direction, with a maximum value observed near Pagra (Km = 1.67 × 10−3 SI). The poor grouping of magnetic fabrics for most impact melt rock samples implies local turbulence in rapidly cooled impact melt at the front of the melt flow immediately after the impact. The mean K1 for most impact melt samples suggests subhorizontal (<5°) flow in various directions. The average value of Km for the target rocks (4.41 × 10−3 SI) is much higher compared to the value for melt breccias (1.09 × 10−3 SI). The results of this study suggest that the melt breccias were likely part of a sheet-like body of sizeable extent. Our magnetic fabric data are also supported by earlier core drilling information from ~70 locations, with coring depths reaching to −500 m. Our extensive field observations combined with available widespread subsurface data imply that the impact melt sheet could have covered as much as 12 km2 in the Dhala structure, with an estimated minimum melt volume of ~2.4 km3.

scholarly journals Design features of the Franz Josef Land volcanic provinces (Arctic Ocean): estimation of lava direction by anisotropy of magnetic susceptibility

2019 ◽  
Vol 486 (2) ◽  
pp. 197-201
Author(s):  
V. V. Abashev ◽  
V. A. Vernikovsky ◽  
A. Yu. Kazansky ◽  
D. V. Metelkin ◽  
N. E. Mikhaltsov ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Simultaneous Action ◽  
Mechanism Of Formation ◽  
Volcanic Province ◽  
Magnetic Fabrics ◽  
Franz Josef Land ◽  
Multi Stage ◽  
First Results ◽  
History Of

The paper presents the first results of studying the anisotropy of magnetic susceptibility in the basalts from Hooker Island, associated with the direction of the melt movement, the location of the eruption centers and the morphology of magmatic bodies. The established features of the primary magnetic fabrics correspond to the trap mechanism of formation of the volcanic province of the Franz Josef Land Archipelago and are a reflection of the simultaneous action of numerous small eruption centers. Previously obtained conclusions about the long, during the Early Jurassic - Early Cretaceous, multi-stage history of magmatism are not confirmed.

scholarly journals Magnetic properties of pseudotachylytes, Jämtland, central Sweden

2019 ◽  
Author(s):  
Hagen Bender ◽  
Bjarne S. G. Almqvist ◽  
Amanda Bergman ◽  
Uwe Ring
Keyword(s):  
Magnetic Susceptibility ◽  
Shear Zones ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Fault Zones ◽  
Small Sample ◽  
Magnetic Fabric ◽  
Normal Faults ◽  
Magnetic Fabrics ◽  
Nappe Complex ◽  
Fault Kinematics

Abstract. Nappe assembly in the Köli Nappe Complex, Jämtland, Sweden, has been associated with in- and out-of-sequence thrusting. Kinematic data from shear zones bounding the Köli Nappe Complex are compatible with this model, but direct evidence from fault zones internally subdividing the nappe complex does not exist. We studied a series of pseudotachylyte exposures in these fault zones for deciphering the role seismic faulting played in the assembly of the Caledonian nappe pile. To constrain the fault kinematics, microstructural and magnetic fabrics of pseudotachylyte in foliation-parallel fault veins have been investigated. Because the pseudotachylyte veins are thin, we focused on small (c. 0.2 cm3) samples for measuring the anisotropy of magnetic susceptibility. The results show inverse proportionality between specimen size and anisotropy of magnetic susceptibility degree, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. This finding implies magnetic anisotropy results acquired from small specimens demand cautious interpretation. However, analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust but yield no kinematic in-formation. Structural field data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Therefore, it is likely that the pseudotachylytes formed late during out-of-sequence thrusting of the Köli Nappe Complex over the Seve Nappe Complex.

Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

2008 ◽  
Vol 146 (3) ◽  
pp. 419-436 ◽  
Author(s):  
M. S. PETRONIS ◽  
B. O'DRISCOLL ◽  
V. R. TROLL ◽  
C. H. EMELEUS ◽  
J. W. GEISSMAN
Keyword(s):  
Magnetic Susceptibility ◽  
Country Rock ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Magnetic Fabric ◽  
Contact Aureole ◽  
Data Set ◽  
The North ◽  
Early Paleocene ◽  
Magma Emplacement

AbstractThe Western Granite is the largest of several granitic bodies around the margin of the Rum Central Igneous Complex. We report palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data that bear on the emplacement and deformation of the Western Granite. The collection includes samples from 27 sites throughout the Western Granite, five sites in adjacent feldspathic peridotite, and two sites in intermediate to mafic hybrid contact aureole rocks. Palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean D = 213.2°, I = −69.5°, α95 = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, −66°, α95 = 4.3°). The discrepancy is eliminated by removing an inferred 15° of northwest-side-down tilting about a best fit horizontal tilt axis trending 040°. Data from the younger peridotite and hybrid rocks of the Rum Layered Suite provide an in situ group mean of D = 182.6°, I = −64.8°, α95 = 4.0°, which is statistically indistinguishable from an early Paleocene expected field, and imply no post-emplacement tilting of these rocks since remanence acquisition. The inferred tilt recorded in the Western Granite, which did not affect the younger Layered Suite, suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the Western Granite to make space for mafic magma emplacement. Magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the Western Granite AMS data set. Group 1 data, defined by rocks from exposures to the east and south, have comparatively high bulk susceptibilities (Kmean, 29.51 × 10−3 in SI system), stronger anisotropies (Pj, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of Kmean (15.89 × 10−3 SI) and Pj (1.014), and triaxial susceptibility ellipsoids. Magnetic lineations argue for emplacement of the granite as a tabular sheet from the south–southeast toward the north and west. Moderate to steeply outward-dipping magnetic foliations, together with deflection of the country rock bedding in the north, are consistent with doming accompanying magma emplacement.

scholarly journals Anisotropy of magnetic susceptibility (AMS) of lava and volcanoclastic flows of the Stephano-Permian Cadi basin (central-eastern Pyrenees)

2020 ◽  
Author(s):  
Ana Simon-Muzas ◽  
Antonio M Casas-Sainz ◽  
Ruth Soto ◽  
Josep Gisbert ◽  
Teresa Román-Berdiel ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
Flow Direction ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Lava Flows ◽  
Magnetic Fabric ◽  
Magnetic Mineralogy ◽  
Thin Sections ◽  
Andesitic Lava ◽  
Magnetic Fabrics ◽  
Standard Specimens

<p>The aim of this work is to apply the anisotropy of magnetic susceptibility (AMS) to determine the primary and tectonic fabrics of lava flows and volcanoclastic materials in one of the Pyrenean Stephano-Permian basins.</p><p>The Pyrenean Range is a double vergence orogen located at the northern end of the Iberian Peninsula. During Carboniferous-Early Permian times the extensional or transtensional regime dominant during the progressive dismantling of the Variscan belt resulted in the development of E-W elongated intra-mountainous basins. This process was coeval with an exceptional episode of magmatic activity, both intrusive and extrusive. The Cadí basin represents a good example of these structures were Stephano-Permian rocks are aligned along an E-W continuous outcrop and reach thickness of several hundreds of meters. The stratigraphy of the study area is characterized by fluviolacustrine sediments changing laterally to volcanoclastic and pyroclastic rocks with interbedded andesitic lava flows.  </p><p>A total of 75 sites (733 standard specimens) were studied and analysed throughout the volcanoclastic, volcanic and intrusive materials of the Stephano-Permian outcrops in the Cadí basin. Samples were drilled in the field along 5 sections with N-S or NW-SE direction in the Grey and Transition Unit. Afterwards, standard specimens were measured in a Kappabridge KLY-3 (AGICO) at the Zaragoza University to characterise the magnetic fabric. The susceptibility bridge combined with a CS-3 furnace (AGICO) was used for the temperature-dependent magnetic susceptibility curves (from 20 to 700 °C) to recognize the magnetic mineralogy. In addition, textural and mineralogical recognition in thin-sections of the samples was carried out in order to recognize the relationship between magnetic and petrographic fabrics.</p><p>The results shows that the bulk magnetic susceptibility of the specimens ranges between 118 and 9060·10<sup>-6</sup> SI but most of the values are bracketed between 160 to 450·10<sup>-6</sup> SI. Taking into account magnetic parameters (Km, Pj and T) there is no correlation between magnetic fabrics and magnetic mineralogy and there is a dominance of triaxial and prolate ellipsoids. Thermomagnetic curves indicate the dominance of paramagnetic behaviour in all the samples and except in one case there is a ferromagnetic contribution due to the generalised presence of magnetite.</p><p>Magnetic ellipsoids can be divided into four main types depending on the orientation of the main axes and associated with the lithologic types: 1) K<sub>max</sub> vertical and K<sub>int </sub>and K<sub>min</sub> horizontal for small intrusive bodies (no restoring); 2) K<sub>max </sub>horizontal or subhorizontal and K<sub>int </sub>and K<sub>min </sub>included in a subvertical plane (before and after restitution) for volcanic breccias; 3) K<sub>min</sub> vertical (after restoring) and three directional maxima for lava flows and 4) non-defined fabric for the explosive materials (probably due to their complex depositional mechanisms). In general, a dominant E-W magnetic lineation is observed in many sites, resulting either from dominant flow direction, or to secondary processes. This is the case for some of the magnetic ellipsoids, that seems to be affected by deformation, K<sub>min</sub> is not normal to bedding and therefore, they do not become vertical after bedding restitution.</p>

Deformation of the Archean Quetico–Shebandowan subprovince boundary in the Canadian Shield near Kashabowie, northern Ontario

1991 ◽  
Vol 28 (1) ◽  
pp. 116-125 ◽  
Author(s):  
Graham Borradaile ◽  
Robert Spark
Keyword(s):  
Strain Analysis ◽  
Canadian Shield ◽  
Vertical Position ◽  
Low Grade ◽  
Northern Ontario ◽  
Southern Boundary ◽  
The North ◽  
Shear Component ◽  
East West ◽  
Compressional Component

The southern boundary of the Quetico metasedimentary subprovince of the Superior Province of the Canadian Shield near Kashabowie, Ontario, is a vertical, east–west feature affected by dextral transpression that had a north-northwest – south-southeast compressional component. A synmetamorphic, locally D1 microfabric and magnetic-susceptibility fabric with an east-directed extension lineation is kinematically compatible with this pattern. It shows the same bedding–cleavage relationship and the same direction of structural facing on D1 in both the Quetico metasediments and the Shebandowan greenstone subprovince on the south side of the Quetico subprovince. In the low-grade rocks of the study area, there is a single phase of penetrative deformation, giving a nearly vertical schistosity at a consistent angle, anticlockwise with respect to the now nearly vertical east–west-striking strata. The absence of penetrative polyphase deformations may be due to the near parallelism of the subprovince boundary with the shear component of dextral transpression. Strain analysis indicates that the minimum shortening of the greywackes is 40% in a north–south direction. It is tentatively suggested that the shortening, the steepening of strata into a vertical position, and some of the S1 fabric development may have occurred prior to the climax of metamorphism and transpression. If this sequence is correct, the strata would have dipped gently to the north prior to the steepening event, with the embryonic schistosity dipping to the west.

Is the Cape Smith Belt (northern Quebec) a klippe?

1985 ◽  
Vol 22 (9) ◽  
pp. 1361-1369 ◽  
Author(s):  
Paul F. Hoffman
Keyword(s):  
Shear Zone ◽  
Granulite Facies ◽  
Magnetic Anomalies ◽  
Ductile Shear Zone ◽  
Tectonic Model ◽  
The North ◽  
Thrust Sheets ◽  
Ductile Shear ◽  
Normal Sense ◽  
East West

The Cape Smith Belt is one of the most interesting and controversial of the proposed geosutures in the Canadian Shield. A new tectonic model is presented in which the mafic–ultramafic thrust sheets of the belt constitute a klippe, 20 000 km2 in area, separated from underlying basement of Superior Province and its thin autochthonous cover by a continuous décollement exposed along the north margin and the plunging eastern end of the belt. Thrusting is directed southward, and the entire stack is folded into a regional antiform (north of the belt) and synform (the belt itself). It is proposed that the décollement is rooted 30–90 km north of the belt, in a zone paralleling Sugluk Inlet, across which there is a major positive deflection in the Bouguer gravity field, an abrupt switch from broad north–south to narrow east–west magnetic anomalies, and a change in metamorphic grade from amphibolite to granulite facies from south to north. In the model, this zone is a north-dipping ductile shear zone juxtaposing crusts of two collided continents. Accordingly, granulite north of the zone represents lower crust of the overriding plate, whereas granulite between the zone and the klippe occurs in antiformal culminations in the underriding plate. One test of the model is that the south-dipping shear zone observed at the north margin of the belt should have a normal sense of slip. Another is that north-dipping banded gneiss at Sugluk Inlet should mark a ductile shear zone, also having a south-directed sense of overthrusting. A structural profile of the belt and its relation to basement can best be worked out by down-plunge projection of the eastern end of the belt.

scholarly journals ANIZOTROPIE MAGNETICKÉ SUSCEPTIBILITY HORNIN NA KONTAKTU METABAZITOVÉ A DIORITOVÉ ZÓNY BRNĚNSKÉHO MASIVU V OKOLÍ VELKÉ BABY U JINAČOVIC

2016 ◽  
Vol 22 (1-2) ◽  
Author(s):  
Lukáš Mareček ◽  
Rostislav Melichar
Keyword(s):  
Magnetic Properties ◽  
Magnetic Susceptibility ◽  
Temperature Dependence ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Small Contribution ◽  
Normal Faulting ◽  
Magnetic Lineation ◽  
Rotational Movement ◽  
Magnetic Fabrics ◽  
Magnetic Foliation

An anisotropy of magnetic susceptibility and temperature dependence on magnetic susceptibility were used to reveal an evolution history along the contact of the Metabasite and Diorite zones of Brno massif north of Brno-Řečkovice. The analysis of temperature dependence of magnetic susceptibility indicated that magnetic properties of all rocks in this area are essentially controlled by magnetite with a very small contribution of pyrhotite and hematite. These minerals were formed later than the primary magmatic minerals. Therefore we assume that magnetic fabrics in studied rocks reflect deformational processes which affected these rocks. There are three patterns in anisotropy of magnetic susceptibility (AMS) in studied rocks. In the first pattern detected in diorites, the magnetic foliation is striking NE–SW, dipping to the NW and there is subvertical magnetic lineation. The second planar magnetic indicates a rotational movement of microgranite rocks along the contact of the Metabasite and Diorite zones. The last pattern found in rocks of the Metabasite zone is magnetic foliation striking NNE–SSW dipping on the NWW and magnetic lineation trending to the SW with plunge of 42° and it shows normal faulting of studied area.

scholarly journals Complexity of the Anisotropy of Magnetic Susceptibility in Single-Phase Deformed, Low-Grade, Cleaved Mudstone

2005 ◽  
Vol 495-497 ◽  
pp. 45-56 ◽  
Author(s):  
Timothy N. Debacker ◽  
Manuel Sintubin ◽  
Philippe Robion
Keyword(s):  
Magnetic Susceptibility ◽  
Single Phase ◽  
Source Area ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Preferred Orientation ◽  
Low Grade ◽  
Sediment Source ◽  
Magnetic Fabrics ◽  
Number Of Factors ◽  
Moesian Platform

The anisotropy of magnetic susceptibility (AMS) is often interpreted in terms of strain. However, since AMS is controlled by all magnetic (s.l.) carriers present, an AMS interpretation is not straightforward, especially in the presence of composite magnetic fabrics. Considering the large number of factors that may influence rock mineralogy (e.g. sediment source area, metamorphism), it becomes clear that one cannot interpret AMS in terms of strain without applying additional techniques that allow determining the nature and preferred orientation of all magnetic (s.l.) carriers likely influencing the measured AMS. This is discussed using samples from the Brabant Massif (Belgium) and the Moesian Platform (E-Romania).

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